Plastics continue to replace metals as materials of construction. The cost savings of plastics over metals combined with the ease of manufacturing and weight savings provide combined impetus for plastic's invasion into the metal field.
Plastics can be fabricated into a variety of geometric designs possessing a wide range of strengths. Indeed, plastics can now be fabricated as replacements for external, semistructural metal members such as automobile hoods, deck lids, roofs and fender extensions. Reinforced thermoplastic molding compounds have shown increased acceptance in recent years due to their lighter weight, lack of outgasing during painting and ease of recyclability. This is in contrast to thermosetting compounds which do not possess the aforementioned attributes.
Of the conventional methods of making plastic parts, including semistructural plastic parts, automatic and semiautomatic molding techniques have proven most popular because of their high production rates, lower manpower costs and improved product uniformity. One of the more popular semiautomatic molding processes is the method known as compression molding. The mold charge is molded upon closing of the mold members as the material flows into the open cavity. Smooth, aligned surfaces, about the perimeter of the male and female mold members, called shear edges, slide together and effect a "pinch off" around the perimeter of the mold to stop the major flow of molding compound from the mold cavity. The edge opening, or shear edge gap, located at the peripheral edges of the mold, is typically on the order of 0.0508 to 0.127 mm (0.002 to 0.005 inch) in width. The gap is purposely kept very small, i.e. just large enough to permit closing of the mold without binding and small enough to prevent any substantial escape of the molding charge from the molding cavity. Unfortunately, in using the small shear edge gap, the surface of such parts, as molded, have an unacceptable appearance and must be hand finished. Wrinkling of the surface is apparent.
During the mold filling step, pressure forces the molding compound to flow and fill out the cavity of the mold. When the molding compound reaches the shear edge, it is very close to the freezing or solidification point of the resin. As the flow impacts the shear edge "wall", the residual energy associated with the flow causes the molding compound to rebound and attempt to flow back into the oncoming flow front, in turn buckling reinforcing material in the molding compound. The buckled reinforcing material becomes frozen in the solidifying matrix resin. As the composition continues to solidify, the buckled reinforcing material begins to show through to the surface of the molded part. This "flow rippling" typically manifests itself as a series of waves with amplitudes as high as 25 microns and a frequency of 1500 to 3000 microns. It generally occurs at or near the shear edges in a compression molded part and is visible to the naked eye both before and after painting. Consequently, products with such surface deformations must be hand-finished prior to painting, which is expensive and time-consuming. Even with hand-finishing, the scrap rate for such molded parts may be as high as 50 percent.
Flow ripples have been documented in several different mold configurations, e.g. round, square and contoured parts. Attempts to solve this problem have been made by altering the resin used as well as changing the reinforcing materials. Statistical testing of the major processing variables showed no significant effect on the magnitude or occurrence of the flow ripples present in the final molded part.